1. Technical Field
The present disclosure relates generally to a method and apparatus for reducing packing ring creep and fluid leakage in rotary machinery and more particularly to rotary bearings and pack-boxes used in low revolutions per minute (rpm) rotary machinery common in the pulp and paper and biofuel industries.
2. Related Art
In industries that use rotary machines—particularly large rotary machines having pressurized journal housings—it can be desirable to separate flush fluid commonly used in journal housings from process fluids commonly used in the body of the rotary machine.
Rotary bearings are usually disposed around the journal and are typically designed to reduce rotational friction caused by the spinning journal. The rotary bearings are generally annularly shaped, made of a material having a low coefficient of friction, and are sufficiently durable to support the weight of the journal. The clearance between the journal and rotary bearing may be filled with flush fluid to lubricate and cool the rotating journal.
A pack-box is a type of journal housing that contains packing rings disposed around the journal. Functional packing rings generally form tight seals around the journal and the pack-box and are generally used to prevent transmission of diffusible matter out of the pack-box. Conventional rotary bearings may rotate around substantially the same axis as the journal and may slide transversely within the pack-box.
Even with the presence of rotary bearings, friction cannot be entirely eliminated. As the journal moves against rotary bearings, sleeve inserts, packing rings, or other internal pack-box components, the journal generates heat. As temperature increases, so does pressure. Pack-boxes typically use packing rings to contain this pressure; however, these packing rings are prone to failure as a result of normal operating conditions. As a result, pressurized flush fluid can leak out of the pack-boxes and interfere with the process the rotary machine was designed to achieve. Flush fluid leaks can also create occupational hazards and shorten the useful life of equipment.
For example, flush fluid leaking into the process can be especially disruptive when the process requires strictly controlled operating parameters such as pH, temperature, or reactant concentration. Leaks can affect product yields and can prevent large-scale manufacturing of products requiring specialized biological agents active at narrow operating conditions. Conversely, leaks from the process into the pack-boxes may expose rotary equipment to hazardous process chemicals and shorten the useful life of the equipment. For example, in pressurized pre-treatment vessels commonly used in the manufacture of lignocellulosic biofuels, steam is often used in the pre-treatment process. This steam mixes with other process chemicals and can be highly corrosive. If the packing rings fail and the steam leaks into the pack-box, the steam will eventually weaken the internal components of the pack-box and render the rotary machine inoperable. When packing rings fail, there is also an increased the risk that the hazardous process chemicals may leak out of the pack-box entirely and expose operating personnel and adjacent equipment.
As the temperature and pressure inside of the pack-box increases, the rotary bearing absorbs heat and expands. A rotary bearing's transverse coefficient of thermal expansion may differ from the bearing's axial coefficient of thermal expansion. That is, a heated rotary bearing may expand parallel to the length of the journal at a different rate than the rotary bearing expands in diameter. While the amount of thermal expansion in the axial directions may be negligible, the transverse expansion can push outwardly against the packing rings. This transverse expansion can compromise the tight seal the packing rings were designed to create, thereby allowing flush fluid to diffuse around the packing rings and out of the pack-box. Operators have attempted to address this problem by using a packing gland follower bolted to the end of the pack-box. When leaks become apparent, the operators may adjust the packing gland follower transversely to press against the packing rings in the opposite direction of the rotary bearing expansion. As the packing rings are transversely pressed, the diameters of the packing rings expand axially to reform tight seals between the journal and the pack-box, thereby stopping leaks.
When the rate of journal rotation slows (e.g. during periods of machine deactivation) the rotary bearing cools and transversely shrinks. The packing rings, however, tend to retain their transversely compressed shape while shrinking axially. The tendency of the packing rings to retain their compressed shape is commonly known as “creep.” Because of creep, a gap is formed between the packing rings and the cooled rotary bearing. If the gap remains present when the rate of journal rotation increases (e.g. when the machine is reactivated), flush fluid can flow into the gap and can provide further pressure on the packing rings as the temperature and pressure inside the pack-box begins to increase. Such pressure can accelerate leaks and packing ring wear. To avoid flush fluid leaks around the edges of the packing ring when the rotary machine is reactivated, operators generally tighten the packing gland follower further to close the gap and compress the packing rings against the cooled rotary bearing. This compression re-forms a tight packing ring seal.
As the temperature inside the pack-box increases, however, the rotary bearing re-expands transversely and further compresses the packing rings. After repeated cycles of machine activation and deactivation, the packing rings reach a threshold at which the packing rings cannot tolerate further transverse compression. If the packing rings are not timely replaced before the packing rings reach this tolerance threshold, the natural rotary bearing expansion resulting from normal rotary machine operation will press against the packing rings and cause the packing rings to fail, thereby dramatically increasing flush fluid leaks and contributing to loss of production.
System operators generally have to deactivate the rotary machinery to replace packing rings, or to remove broken packing rings. Replacement packing rings can be costly because packing rings are generally made from complex synthetic materials designed to withstand the high pressure and temperature common in pressurized pack-boxes. Accordingly, there is a need for an improved apparatus and method for reducing flush fluid leaks in pack-boxes for rotary machines while reducing rotary bearing and packing ring wear over conventional designs.